ELECTROMAGNETIC WAVES AND THE

ELECTROMAGNETIC SPECTRUM

Unit 32

Unit 32 Electromagnetic Radiation Electromagnetic waves

perpendicular waves combing the electric and magnetic fields.

Electromagnetic waves are produced when an electric charge vibrates or accelerates and can travel through empty space or through matter. 

Fig. 1. Electromagnetic spectrumThe electromagnetic spectrum is made up of many different types of electromagnetic waves. The frequency and wavelengths of these waves determine the type of electromagnetic wave. Figure 1 shows the electromagnetic spectrum and its waves.

Which of the following is NOT an example of an electromagnetic wave?

A. X-ray B. Microwave C. Visible light D. Sound wave

Which of the following is NOT an example of an electromagnetic wave?

A. X-ray B. Microwave C. Visible light D. Sound wave

Behavior and Color of Light   Interactions of Light   When light strikes a new medium or

substance, the light can be reflected, absorbed, or transmitted. When light is transmitted it can be refracted, polarized, or scattered.

Scattering and Refraction

Why is the sky blue?

Scattering and Refraction

Have you ever wondered why the sky is blue during the day and reddish orange at dusk?

It is due to the scattering of the light rays (REFRACTION).

The sunlight gets scattered by the molecules of the atmosphere.

Particles in the atmosphere scatter shorter wavelength blue light more than the longer wavelength red light.

Light Paths in our Atmosphere

Red light goes straight through the atmosphere.

Blue light scatters all over.

Thus, you see the sky as blue. If blue light did not scatter, the sky would be black

Sunset in our Atmosphere

Larger particles like dust and water droplets reflect red light causing dusk and dawn to look red and orange.

Refraction

Refraction is the bending of light that is caused by a change in speed when the light wave goes from one medium into another.

Figure 32.2.2 demonstrates this concept.

Refraction

Refraction is the bending of light that is caused by a change in speed when the light wave goes from one medium into another.

That is why things appear to be somewhere they are not underwater.

Reflection

Reflection of a light wave occurs when the light wave bounces off an object.

The Angle of Incidence is equal to the angle of reflection.

These angles are always measured from a line perpendicular to the surface.

Scattering

Light Producers

An object that produces light is called a luminous object. Luminous objects include the sun, light bulbs, lightning, and hot objects, etc.

All objects produce electro-magnetic waves.

Most produce low frequency radiowaves. Only very hot or energetic objects produce wavelengths that can be seen by our eyes.

Light Reflectors

Most objects that we see with our eyes do not produce their own light and must rely on the reflection of light in order for us to see them.

For example, as light hits your shirt the color your eye detects is the color of light that has been reflected or re-emitted by your shirt.

Most of the other colors were absorbed by the materials.

Light Sources

Objects that give off their own light are said to be luminous. Some common luminous objects that you are aware of are the sun, incandescent light bulbs, fluorescent light bulbs, lasers, and neon lights.

Glowing Objects

Incandescent light is light that is produced as a result of a glowing hot object.

Light bulbs that have filaments in them have an electric current pass through them. As a result of this electric current, the filament heats up and begins to glow, giving off light.

Flourescence

Fluorescent light is created by a process called fluorescence. In a fluorescent light there are two electrodes, one on each end of the tube, phosphor powder, and mercury vapor.

The electric current comes in at one electrode and transmit the electrons into the gases.

The mercury atom electrons get excited, jump to a higher energy level, and then drop back down to their original energy state. When the electrons return to their original state they emit photons, seen as visible light of all different frequencies and wavelengths.

Mercury

Fluorescent bulbs do not get very hot and they are more energy efficient than incandescent bulbs;

However, the proper disposal of a fluorescent light bulb requires attention because it contains toxic mercury.

A fluorescent bulb that has burned out should be brought a recycle center.

To find a recycle center near you, go to Electronics: Earth 911 and type in fluorescent light bulbs and your zip code in the top.

Electromagnetic Waves

Characteristics of Electromagnetic Waves Transverse Waves Alternating electric and magnetic fields Characteristics of waves and particles All travel at 3.0 x 108 m/s (Speed of Light) It takes 8 minutes for light to get from the

Sun to Earth It takes 3 seconds for radio signals to get

from Earth to the Moon

Electromagnetic Spectrum

Includes the following types of waves Radio Microwaves Radar Infrared Visible Light Ultraviolet X-Rays Gamma Rays

Radio and Microwaves

Radio waves are used for radio, cell phones, remote controlled devices, GPS/satellite communications, and television.

Microwaves are the shortest wavelength radio waves and are used in many communication functions.

Radar is used to determine how fast an object travels. It incorporates the use of radio signals, receivers, and the Doppler effect in order to determine the speed of an object.

Infrared Waves

Infrared waves are heat waves. All objects emit infrared waves because all

objects possess heat. Night vision goggles detect infrared waves and

allow the user to see the movement of objects in the dark.

Many TV remotes use infrared.

Infrared image of a human body. Red is hot, blue is cool.

Visible Light

Our eyes detect electromagnetic energy in a small portion of the electromagnetic spectrum called the visible light region.

The visible light region corresponds the wavelengths and frequencies of red, orange, yellow, green, blue, indigo, and violet light.

Ultraviolet Waves

UV Light that is at a higher frequency and energy than violet light.

UV light can kill micro-organisms. Too much exposure can cause:

Sunburn, Wrinkles, and skin cancer As it damages cell DNA

X-Rays

X-Rays have high energy and can penetrate some material. Used in:

Medicine Industry Transportation

Too much exposure can damage living tissue or even cause cancer.

Gamma Rays

Gamma rays are the highest energy electromagnetic wave.

They usually come from radioactive elements or stars Uses include:

Killing cancer cells Making pictures of the brain Inspection tools in industry

Speed of Electromagnetic Waves

All electromagnetic waves travel at 3.0 x 108 m/s in a vacuum.

Speed of a wave formula: Speed = Frequency x Wavelength

As the frequency increases, the wavelength decreases.

See the next slide for a diagram of this relationship.

Electromagnetic Spectrum Diagram

Diagram of Electromagnetic Spectrum (including the relationship between frequency and wavelength).